1. Field of the Invention
This invention relates in general to the construction of valves and in particular to a new and useful vacuum metering valve which includes a mechanical adjustment for setting a valve member in relation to a valve seat and which has means associated therewith for varying the position of the valve member in relation to the seat by varying the temperature or in accordance with the temperature variations.
2. Description of the Prior Art
Vacuum metering valves are needed in the vacuum technique for many purposes, particularly in two applications:
A. Where it is necessary, in a vacuum process, to continuously supply an exactly metered quantity of a precipitating gaseous reagent, for example, in a reactive deposition of thin layers, in this case, the valve is adjusted to a predetermined conducting valve; and
B. In connection with closed-loop control devices for keeping the pressure constant in a vacuum space which is continuously evacuated and simultaneously continuously supplied, through the valve, with exactly the same amount of gas in order to maintain the predetermined pressure.
Valves for these purposes are frequently designed as so-called "needle valves" which, however, have the disadvantage that a sticking (cold welding) easily occurs between the needle tip serving as the closing member and the valve seat. Since in a needle valve, the passage opening is small, it may also easily become clogged with dirt.
That is why, recently, another construction has been used more often in which a valve plate of a hard material, for example, sapphire, is pressed against a valve seat designed as a knifeedge of softer material, for example, copper. Such valves can be better cleaned, have a longer life and may be made heatable in which case temperatures up to 450.degree. C. are used in order to obtain a secure degassing of all parts coming into contact with the vacuum, which is important particularly in high vacuum applications. In such valves, however, while closed, so strong forces may be produced between the valve closing member and the valve seat, due to the unequal thermal expansion, particularly, of the housing and the valve rod, that the sealing edge is deformed excessively and the valve is either completely destroyed or at least its life is shortened. To avoid such consequences, devices for compensating the expansion have already been provided to counteract the occurrence of too strong forces by an elastic deformation of individual component parts of the valve, for example, resilient valve disks. Such special constructions are expensive and in addition, the drawback is always present that the passage opening of the valve, which, for metering the gas supply, is adjusted to a predetermined conducting value, also varies with the variation of the ambient temperature so that repeated readjustments are necessary and, moreover, a separate calibration is needed for the valve for every temperature.
Another disadvantage of all known metering valves to be adjusted by means of a screw or another mechanical or electromagnetic mechanism is that they must be manufactured with the highest precision in order to permit a sufficiently exact adjustment of the gas passage, particularly with small rates of flow.
Further known are gas metering valves in which a closing member is received in a tube, and tube and closing member are made of materials having unequal thermal coefficients of expansion. The opening or a change of the conducting value is effected so that upon heating the whole arrangement, the narrow gap between the closing member and the inside surface of the tube varies as a result of the unequal expansion of these two parts. In such cases, however, temperatures up to several 100.degree. C. must be used and the device has a further disadvantage in that because of the thermal capacity of the parts to be heated, each adjustment to a definite gas conducting value is accomplished only in short increments so that such thermal metering valves can hardly be employed for closed-loop pressure control devices.
Also known are valves for liquids in which the thermal expansion of the volume of an elastic hollow body filled with liquid or gas and connected to a heating device, is used for the flow control. Such valves are used, for example, in heating installations. Bimetal-controlled devices for the flow control in lines are also known.